In reading through that I noticed the discussion around Fig. 4, on sailing downwind faster than the wind by using the wings in a windmill configuration.

That made me wonder why wing-and-wing is not faster than the wind velocity. Is it the flexible sails? Or the drag of the hull, perhaps. so I kept looking and then I found this video, which finally helped me understand a lot of the rest of this discussion. I think I just needed to see it from another direction, and this did it:

In reading through that I noticed the discussion around Fig. 4, on sailing downwind faster than the wind by using the wings in a windmill configuration.

That is a quite misleading name for the DDWFTTW operation, they use there. It fits for the directly upwind case. But for DDWFTTW the wings are actually in "propeller mode". When you look at the lateral (perpendicular to Vs) components of F and Vm, you see that the wing moves opposite to the aerodynamic force along the lateral axis. This is analogous to a propeller which rotates opposite to the aerodynamic torque. A windmill(turbine) rotates according to the aerodynamic torque.

Thank you for these thought provoking documents. It will take awhile to comprehend their implications to a TI type of sailing craft so the question I'm about to ask is admittedly half baked.

But it seemed from my reading of Bauer's windsail design that in the windmill mode the equivalent of the water prop's thrust powered by the rotary windmill above was the keel's thrust powered by the sail riding above the keel. The symmetry of the two sails and two keels was such that they canceled the lateral forces by tacking simultaneously always in opposite directions. The novel idea was separating the tacking motion from the boat's motion.

It wasn't clear if the keel thrust was the only thrust with nothing from the sail when in windmill mode since he seemed to indicate that it could also work in some combination (windmill or prop equivalent) as a function of heading.

But let's assume it is 100% in windmill mode with the keel as the only forward thrust then it seems that there may be other methods for canceling lateral forces and contributing forward thrust that involve opposite tacks but not necessarily from laterally spaced keels with their engineering support problems.

It seems there is not much research on the hydrodynamics of undulatory propulsion with respect to modelling both the cancellation of lateral forces in dense media like water but also converting a component of those forces to forward thrust like fish body propulsion (not fins). But such propulsion may offer an integrated compact way of simultaneously cancelling lateral forces and providing forward thrust all within a small space since the propagating wave is longitudinal in the direction of travel rather than lateral.

Then in windmill mode one could imagine the windmill above the fish (looks like a sail fish) with multiple sails (webs) and masts (spines) which power the submerged undulations and the lateral force cancellations happen in sequence as the wind drives sail waves off multiple sails propagating down the spine which oscillates in waves from one tack to the opposite tack with the undulatory fish propulsion below mimicking the action above and providing both the cancellation forces and the forward propulsion.

There are variations of this idea with only a few or even two sails with one in front of the other and connected in sequence in the same direction of travel and driving the undulatory propulsion below.

My question is: since you seem to have access to research articles in the esoteric field of nontraditional sailing, do you know of research using undulatory propulsion powered by wind above in a windmill mode sailing craft?

Actually, my interest is in the application of nature to practical real world products. Hobie invented the mirage drive which many believe is more useful and flexible than a propeller version by another company e.g. more robust, sheds weeds better, easily retracts in shallow water and seems more efficient for thrust. See http://www.hobiecat.com/forums/viewtopic.php?f=78&t=33081&start=0 for that discussion.

That very appealing feature of Hobie TI/AI and kayaks ultimately leverages off the research on fin propulsion in nature. It is in that sense I'm interested in understanding the lessons from nature that could lead to improvements to the TI. Understanding windmill powered boats and DDWFTTW and its related research including fin and even undulatory propulsion as substitutes for propellers may also contribute to TI improvements.

Many of us live in poor sailing environments such as inland small lakes. The best winds are in spring and fall when it's too cold to enjoy sailing an open wet boat like the TI. The good summer winds are only when fronts come through and too often are too gusty. The rest of the time the wind is very low and from widely varying directions since local thermals dominate. Any technology that results in going from 3 to 6 mph in that lousy wind would be exciting.

One such technology may be substituting the center board with a horizontally long, semi-stiff but flexible fin that not only acts as a traditional center board but also converts the varying lateral forces into forward thrust components much like the undulatory movements of a fish body. This would work best in those weak summer winds of varying directions from random thermals and may increase the speed by better exploiting the changing wind directions. It may also be possible to actively produce undulation induced thrust on that center board fin by harnessing some of the varying wasted lateral forces on the main sheet.

The TI is a particularly good boat to experiment with since it is easy to retract the existing center board and try out various fin like center boards through one of the mirage drive wells.

For going into the wind the air-rotor is a turbine, while the underwater-rotor is a propeller.

In that case it seems that there is a clean function for each physical thing without overlapping but not necessarily for other sailing devices.

Is there always a clean distinction between turbine mode of energy capture and propeller function of thrust for a given physical device such as an air-rotor or sail?

How does that work in a conventional sail boat? Which is the turbine capturing the energy and what is the propeller equivalent providing thrust?

Could the same physical thing be both turbine and thruster simultaneously? Does the sail capture energy that drives the load of the boat and keel which nets forward lift? Does the sail also provide lift and so forward propulsion from the relative speed of the boat versus air regardless if that boat speed came from motor below, from the forward lift of the keel or a mirage drive?

For DDWFTTW is the air-rotor only a propeller (providing thrust) and the wheels only an energy capture device powering the load of the propeller? Or is the air-rotor also capturing energy and the wheels also a load at times? Otherwise how does it get going and later how does it slow down when the tail wind shuts off?

Is there always a clean distinction between turbine mode of energy capture and propeller function of thrust for a given physical device such as an air-rotor or sail?

For a rotor there is a clean general distinction:

propeller : rotates opposite to the torque from the fluidturbine : rotates in accordance with the torque from the fluid

There are other critera based on energy transfer, but those unlike the above are frame dependent and fail in special cases.

bobco wrote:

How does that work in a conventional sail boat? Which is the turbine capturing the energy and what is the propeller equivalent providing thrust?

The energy question is frame dependent. But we can translate the above distincion for rotors to the a sail boat:

The rotation becomes linear motion perpendicular to the true wind vector. The tangenial componenets of the rotor blade become the true-wind-perpendicular -components (TWPPC). Considering only those components of the velocity & force vectors we can decide if the sail or the keel act as a propeller or turbine blade:

propeller : velocity-TWPPC opposite fluid-force-TWPPCturbine : velocity-TWPPC in the same direction as the fluid-force-TWPPC

Applying this to a sail boat gives different answers depending on the situation:

For DDWFTTW is the air-rotor only a propeller (providing thrust) and the wheels only an energy capture device powering the load of the propeller? Or is the air-rotor also capturing energy and the wheels also a load at times? Otherwise how does it get going and later how does it slow down when the tail wind shuts off?

As I allready noted: Energy considerations are not usefull to describe how moving things powered by the relative movement of two other things work. Since kinetic energy is frame dependent different observers will disagree which thing provides energy. This can get very confusing:

In the ground frame the DDWFTTW air-rotor always takes energy from the air. In the vechicle's frame it takes energy from the air below windspeed, and gives it to the air above windspeed.

This is much simpler:

The air-rotor of the DDWFTTW is always a propeller because it always rotates opposite to the aerodynamic torque.

Wow! This topic keeps cropping up. The last time I saw it, it was a group of my old hang gliding buddies. And there are always a group of scientist-wannabes, an occasionally a physic professor, that gets it wrong.

It is possible. It does not violate the law of conservation of energy. It does not violate the law of conservation of momentum. For many people, it does violoate the law of "but that doesn't make sense to me" or the law of "I don't get it". Those are not laws of physics.

The energy to keep the cart moving or to accelerate it is taken from the kinetic energy of the wind, by slowing the air/wind down. When the cart is moving faster than the wind, the air to be slowed down is air that the cart hasn't overtaken yet. The trick here is that as the cart overtakes the air moving in the same direction, that air does NOT hit the FRONT of the turning blades. It still hits the back of the blades, and can and does push the car forward. The air itself is pushed in the direction opposite to the wind.

This does NOT mean that you can steal energy from nowhere, nor create a perpetual motion machine, nor ...

I have created a detailed, mathematical treatment of the problem. It can be found at

The energy to keep the cart moving or to accelerate it is taken from the kinetic energy of the wind, by slowing the air/wind down. When the cart is moving faster than the wind, the air to be slowed down is air that the cart hasn't overtaken yet. The trick here is that as the cart overtakes the air moving in the same direction, that air does NOT hit the FRONT of the turning blades. It still hits the back of the blades, and can and does push the car forward. The air itself is pushed in the direction opposite to the wind.

The above is shown in this animation ("ground frame" part). Note the red tracer that indicates how an air molecule is slowed down relative to the ground by the collision with the blade:

tadhurst wrote:

BTW, that question is also answered by several people that have built an actual cart that goes DWFTTW - physical evidence!